1. Field of the Invention
This invention relates to a valve motion for an internal combustion engine wherein the valve operation characteristic of an engine valve can be changed.
2. Description of Background Art
A valve motion for an internal combustion engine is known wherein a camshaft rotates in synchronism with the rotation of the internal combustion engine with a driving cam that rotates integrally with the camshaft. A valve motion cam makes relative rotation to the camshaft to open and close an engine valve. A link mechanism for transmitting a valve driving force of the driving cam to the valve motion cam is provided. A holder member supports a fulcrum of the link mechanism thereon and is rockable around the camshaft. A driving mechanism for rocking the holder member to vary the position of the fulcrum of the link mechanism is provided wherein a valve operation characteristic in accordance with which the engine valve opening and closing can be changed by the rocked position of the fulcrum of the link mechanism. This driving mechanism includes a slider for rocking the holder member and a feed screw shaft extending in an upward and downward direction of the internal combustion engine for moving the slider and for moving the slider on the feed screw shaft to rock the holder member. See, for example, Japanese Patent Laid-Open No. 2005-207254.
However, with the conventional configuration described above, since the feed screw shaft for moving the slider extends in an upward and downward direction of the internal combustion engine, the height of the internal combustion engine is increased. Thus, it is difficult to incorporate the valve motion in a small-sized vehicle such as a motorcycle.